A healthy heart responds to stress and activity such as exercise by increasing cardiac output through increased heart rate and stroke volume. Optimally, a pacemaker would mimic this natural response in all conditions where increased cardiac output is necessary. Conventional fixed-rate pacemakers can provide some degree of exercise tolerance because the myocardium is sometimes healthy enough to increase cardiac output by increasing stroke volume. However, not all patients have an adequate response with stroke volume increases alone. Increasing the pacing rate during exercise significantly increases cardiac output and, consequently, exercise tolerance.
Atrial-triggered pacemakers, also known as P-wave triggered pacemakers and P-wave synchronous pacemakers, theoretically provide the optimal rate response. They sense atrial electrical depolarizations, commonly observed as "P-waves" on an electrocardiograph (EKG), with an electrode in the atrium and pace the ventricle after an appropriate AV delay period that approximates the AV conduction time. These pacemakers help patients tolerate exercise, because the ventricular pacing rate follows the atrial rate, which increases with exercise. Atrial-triggered ventricular pacing is incorporated into the DDD pacemaker, which paces and senses both chambers. Ventricular pacing is triggered after the AV delay period by a sensed atrial event and inhibited by a sensed ventricular event. Atrial pacing is inhibited by a sensed event in either chamber.
The principle of P-wave sensing and delayed ventricular stimulation is straightforward, but its implementation has led to several problems that have yet to be completely overcome. First, the atrial rate is not always a reliable indicator of the optimal pacing rate. Atrial bradycardia and, more importantly, atrial tachycardia occur in some pacemaker patients, and pacing the ventricle at these abnormal rates must be avoided. Second, atrial flutter and fibrillation also render atrial-triggered stimulation detrimental. A third major problem arises in cases of retrograde AV conduction, which is present in an estimated one-third of patients with complete AV block and two-thirds of patients with lesser degrees of block. Pacemaker-mediated tachycardia (PMT) can occur in such cases because a ventricular pulse is conducted from the ventricular electrode along the retrograde conduction path to the atrial electrode where it is sensed as an atrial contraction. The atrial-triggered pacemaker responds by generating another ventricular pulse after another AV delay period. If not prevented, pacemaker-mediated tachycardia can be lethal.
These problems have prompted several modifications of atrial-triggered pacemakers. Maximum ventricular pacing rates are commonly limited so that, in the presence of atrial flutter, fibrillation, or tachycardia, ventricular pacing will either be maintained at a maximum rate or reduced (sharply or slowly) to a minimum rate. Both options sacrifice exercise responsiveness to maintain the ventricular rate within safe limits. Some doctors have sought a balance by setting the upper rate limit high enough to provide some exercise tolerance and yet low enough to preserve some degree of immunity from retrograde conduction. While this approach may be satisfactory in some cases, it is generally undesirable because any compromise introduces some risk of pacemaker-mediated tachycardia.
Two alternative methods of responding to high atrial rates are 2:1 and Wenckebach heart block. These methods involve controlling the atrial refractory period to inhibit sensing of atrial electrical depolarizations above a set maximum atrial rate. In the 2:1 block method, every other "P-wave" is ignored above a maximum atrial rate. U.S. Pat. No. 4,467,810 to Vollmann shows a pacemaker operable in the DDD mode with 2:1 block. Here, the atrial refractory period is set equal to a time interval or period corresponding to the desired maximum atrial rate. Consequently, when the intrinsic atrial rate exceeds the maximum rate, every second "P-wave" will occur during the atrial refractory period and not be sensed. The maximum atrial rate limits the patient's ability to exercise. A patient with a retrograde VA conduction path can suffer PMT when the maximum atrial rate is set for high exercise tolerance, because the atrial alert period is proportionally longer for high rates. Therefore, the probability is higher that artifacts of ventricular pulses will appear at the atrial electrode during the atrial alert period.
The Wenckebach method consists of progressively delaying the ventricular stimulus, effectively increasing the AV delay period until intermittent block occurs, thus lowering the average ventricular rate. The beat-to-beat ventricular rate is erratic, but the average ventricular rate is higher than during 2:1 block.
Modifications of the atrial-triggered pacemaker have not fully overcome its limitations. Pacemakers are being developed which employ an indicator other than the atrial rate to determine metabolic need. Proposed indicators of activity include nerve action potential frequency, Q-T interval, hydrogen-ion concentration (pH), venous oxygen saturation, respiratory rate, stroke volume, body motion, and body temperature. Proposed pacemakers responsive to these indicators are discussed in an article entitled, "Principles of Exercise Responsive Pacemakers" by Fearnot, Geddes and Smith, on pp. 25-29 of the June, 1984, issue of IEEE Engineering in Medicine and Biology Magazine. Problems with sensitivity, accuracy, and transducer reliability and power consumption impede the practical implementation of some of these techniques. Moreover, optimal pacemaker response is difficult to attain because physiological indicators which vary in response to exercise and stress often exhibit similar variations in response to other conditions not affecting cardiac output requirements. Algorithms have been designed to differentiate between certain true and false indications of exercise, but such algorithms are generally complex because responses to many conditions vary widely from patient to patient. Furthermore, algorithm efficacy is difficult to verify, particularly in human tests, and an algorithm designed to solve one problem may actually introduce others.
The pacemaker shown in U.S. Pat. No. 4,436,092 to Cook et al. controls the heart stimulation rate according to an algorithm relating heart rate to right ventricular blood temperature. It generates stimulation pulses at a demand rate in the absence of natural cardiac activity occurring at a higher rate. The responsiveness of this pacemaker to exercise has been demonstrated, yet its control algorithm does not change pacing rate limits as a function of exercise level.
Bornzin, in U.S. Pat. No. 4,467,807, discloses a rate-adaptive demand pacemaker in which the level of oxygen within intracardiac or pulmonary-artery venous blood determines the escape interval for demand pacing. As such, a given minimum rate is determined for a given oxygen level. FIG. 5 of the Bornzin patent shows an atrial-triggered pacemaker incorporating this rate-determining technique. Although the minimum rate is adjustable, no provision for maximum rate behavior is disclosed.